Method of drying wood



Patented Dec. 2a, 1950 METHOD OF DRYING WOOD Monle 8. Hudson, Spa'rtanburg, S. C.

No Drawing. Application in. :0,- 1m,

Serial No. 602,888

This invention relates to drying wood, and more particularly to the drying of wood with organic vapors.

According to my copending application, Ser.

No. 579,851, filed February 26, 1945, now Patent No. 2,435,218, green or wet wood may be dried in a particularly rapid and emcient manner by treating the wood in a closed space with a highly heated organic vapor that is inert to the wood and that permeates the wood so as to flash-oft moisture in the wood as vapor. This process is carried out by subjecting the wet wood to the drying treatment in the closed drying space with organic vapor maintained at a temperature suificiently high to liberate rapidly the moisture in the wet wood, at a pressure obtainable in the presence of the wet wood, while continuously removing liberated moisture vapor and spent organic vapor from the closed drying space.

In employing the drying process disclosed in my above identified prior patent, it has been.

found that internal checking is induced when wood, particularly certain ring porous hardwoods, are dried according to this method for extended periods; that is, for periods in excess. oi about three hours. The ring porous hardwoods are characterized by annual rings having relatively large pores in the spring wood as compared with the summer wood. Certain of these woods are also characterized by the presence of relatively wide medullary rays. Representative species are: oak, beech and locust.

The internal checks induced by extended dry- 5 Claims. (Cl. 34-465) In view of this fundamental diflerence in the mechanism of drying in the case of the two types of wood, it appears that the pronounced internal checking effectin the ring porous hardwoods is due to the lowering of the moisture contentin the outer sections ot-the wood too far below the fiber saturation point before sutlicient drying has been eilected in the inner sections of the wood. It may also be that the cleavage planes afiorded by the wide medullary rays in certain of the ring porous hardwoods favor internal checking.

In some instances, internal checking is advantageous because it relieves the stresses set up in the wood during the drying process, and in this manner functions to obviate subsequent external checking. It has been found in actual practice, for instance, that vapor dried cross ties in which internal checking had developed during vapor drying showed considerably less external checking in service than comparable air seasoned ties. However, where it is necessary tojdry green or wet wood to a relatively low final moisture content, so that the drying process has to be carried out for a relatively long period, the internal checking may become excessive and it is desirable to be able to control this effect. Also, this internal checking reduces the mechanical strength of the dried wood mg begin in the outer sections of the wood, usually a about A to inch below the surface, and ,ex-. tend in toward the center of the wood for a distance of perhaps 1 to 1% inches. The checks are very narrow in width, and usually appear as hair lines. They follow the medullary rays, and in a given cross-section practically every medullary ray is apt to be affected.

l The ring porous, and particularly the wide rayed ring porous, hardwoods difler markedly as regards internal checking from the diil'use'porous hardwoods, such as the various gum species. The

latter do not develop any substantial amount of internal checking as a result of extended drying, even for periods of twelve hours or longer. This difference is apparently due to the fact that the difluse porous structure allows redistribution or moisture to occur more freely than is possible in the ring porous types of wood; that is, transfer of water from wetter to drier areas occurs more freely in the diffuse porous than the ring porous types or wood.

somewhat, so that where the wood is intended for use under circumstances which protect it from the weather and other conditions which tend to produce external checking, it is dedsirable to avoid internal checking during the drying process.

I have now found that internal checking in these ring porous hardwoods and in other woods can be precluded by carrying out the drying process with alternatingperiods of vapor drying and vacuum. According to the present invention, wood to be dried is initially subjected to the vapor drying treatment as disclosed in my above mentioned prior patent, in which a detailed description of the manipulative steps for carrying out a the vapor treatment is given.

Briefly, the procedure includes the steps oi! placing the wood to be dried in a closed space and introducing into this space vapors of an organic liquid having a boiling point within the range from about 260 F. to 450 F., so that the vapors come into intimate contact with the wood and raise the temperature thereof above the boiling point of water. Thus, the water in the wood is vaporized and may be removed continuously from the closed space with the spent organic vapors to a condenser and thence into a separator where the water and organic liquid are separated.

The condensed water is measured as it is discharged so that there is available a constant check on the amount of water that has been removed from the wood. When the desired amount 01' moisture has been removed, the vapor treatment is discontinued. If desired, the dried wood may be subjected to a final vacuum period before removing it from the closed drying space to recover organic drying medium that may have been absorbed by the wood during the vapor treatment.

According to the present invention, the amount of moisture to be removed from the wood before the vapor treatment is discontinued and the vacuum treatment is begun, is less than the amount that would result in lowering the moisture content of the outer sections of the wood below the fiber saturation point. The exact amount of moisture removal that will result in lowering the moisture content of the outer section of the wood to the fiber saturation point will, of course, be dependent upon the particular type of wood, the moisture content of the wood, and upon the rate of moisture removal from the wood. For each wood, however, and under each specific set of operating conditions, the amount of moisture removal that will correspond to reduction of the moisture content in the outer sections to the fiber saturation point can be determined by standard techniques, having in mind that the moisture removal comes substantially entirely from the outer sections of the wood until the moisture content there is reduced substantially'below that of the inner sections.

In all cases, according to the present invention, the initial vapor cycle is discontinued before the moisture content of the outer sections of the wood being dried has been reduced substantially below the fiber saturation point. The wood is then subjected to vacuum as described in the above-mentioned prior patent for a period of the order of the duration of the initial vapor drying treatment, as described more in detail below, during which it appears that redistribution of the moisture in the wood from the inner sections to the outer dried sections takes place. after which the organic vapor drying and vacuum treatments are repeated alternately until the wood has been dried as desired.

As mentioned above, internal checking develops in the ring porous hardwoods when they are dried in excess of about three hours, which corresponds in most cases 'to the degree of drying at which the moisture content in the outer sections of the wood being dried is reduced below the fiber saturation point. Each period of vapor drying is consequently carried out according to the present invention within this limit. Periods of vapor drying of as long as three hours may be satisfactorily employed, and in the case of some species the period may perhaps be extended somewhat, but a shorter period is preferably used. Also, it is desirable to approximately balance the total periods of vapor drying and vacuum treatment.

In actual practice I have found that an alternating cycle of about one hour of vapor drying and one hour of vacuum repeated a sufllcient number of times to eii'ect the degree of drying desired produces very satisfactory results. Internal checking is inhibited in wood dried in this manner and although the vapor drying process is rendered somewhat slower than with a straight drying cycle, the alternating cycle drying process is still much more rapid and eflective than other prior art methods.

The above described 1:1 ratio 01' total vapor drying time to vacuum may be varied to about 2:1 in either direction ii desired. It the imbalance of vapor drying time to vacuum is any greater, however, the mechanism of drying does not appear to be modified properly to inhibit the internal checking, and further variations are not recommended.

While my invention has been described in terms of the vapor drying process and apparatus described in my above mentioned prior patent and is particularly applicable for use in connection with the apparatus and method described in that application, it will be understood that the present invention is of general utility and may also be employed in connection with other comparable methods 01' drying wood with organic vapors.

I claim:

1. The process of drying wood with organic vapors which comprises subj cting the wood in a closed space to an initial drying period by introducing into said closed space vapors of an organic liquid maintained at a temperature sumciently high to liberate rapidly as water vapor the moisture in the wood while continuously removing liberated water vapor and spent organic vapor from said closed space, discontinuing the introduction of said organic vapors when the amount of moisture liberated from the wood indicates that the moisture content of the outer sections of the wood being dried has been lowered to the fiber saturation point during said initial drying period, subjecting the wood to vacuum following said initial drying period by impressing a vacuum on said closed space for a period of the order of the duration of the initial vapor drying period annd thereby allowing redistribution of the moisture remaining in the wood from the inner sections to the outer dried sections, and then alternately repeating said organic vapor drying and vacuum treatments until the wood has been dried as desired while avoiding further drying of the outer sections of the wood too far below the fiber saturation point before suflicient I drying has been eilected in the inner sections as water vapor the moisture in the wood while continuously removing liberated water vapor and spent organic vapor from said closed space, subjecting the wood to vacuum following this initial vapor drying treatment by impressing a vacuum on said closed space for a period of not more than about twice the period of vapor drying and not less than about one-half said period, and then alternately repeating said organic vapor drying 'and vacuum treatments until the wood has been dried as desired.

3. The process defined in claim 2 and further characterized in that the vacuum following each period of organic va or drying is maintained for a period substantially equal to the vapor d17- ing period.

4. The method of inhibiting internal checking during the drying of wood with organic vapors which comprises subjecting the wood in a closed space having exhaust and supply ports to an initial period of vapor drying by introducing into said closed space through said supply port vapors of an organic liquid maintained at a temperature sufficiently high to liberate rapidly as water vapor the moisture in the wood while continuously removing liberated water vapor and spent organic vapor from said closed space, discontinuing the introduction of said organic vapors by closing said supply port when the amount of moisture liberated from the wood indicates that the moisture content of the outer sections oi the wood being dried has been lowered to the iiber saturation point during said initial drying period, subjecting the wood to vacuum immediately following said initial drying period by impressing a vacuum on said closed space through said externately repeating said organic vapor drying and 3 vacuum treatments until the desired degree of drying has been eflected while avoiding further drying of the outer sections of the wood too far below the fiber saturation point before sufllcient drying 2 has been effected in the inner sections of the wood.

5. In the drying of green or wet wood in a closed space with a highly heated, inert, organic vapor maintained at a temperature sufllciently high to liberate rapidly as water vapor the moisture in the wood at a pressure obtainable in the presence of the wet wood, the method of inhibiting internal checking of the wood during drying which comprises initially heating the wood to be dried with said organic vapor in said closed space while continuously removing liberated water vapor and spent organic vapor from said closed space, discontinuing said heating with organic vapor when the amount of moisture liberated from the wood indicates that themoisture content of the outer sections of the wood being dried has been reduced to the-fiber saturation point, subjecting the wood in said closed. space to vacuum for a period of not more than about twice the duration of the initial heating period and. not less than about one-half said period to allow redistribution of the moisture in the wood from the inner sections to the outer dried sections, and then alternately repeating said organic vapor heating and vacuum treatments until the desired degree of drying has been effected while avoiding further drying of the outer sections of the wood too far below the fiber saturation point before suflicient drying has been efiected in the inner sections oi the wood.

MONIE S. HUDSON.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 763,482 Gray June 28, 1004 827,730 Jones Aug. 7, 1906 1,004,168 Howard Sept. 26, 1911 1,059,820 Besemfelder Apr. 22, 1913 1,166,819 Derby et al. Jan. 4, 1916 1,219,406 Banks Mar. 13, 1917 1,328,505 Fish, Jr Jani20, 1920 1,328,506 Fish, Jr Jan. 20, 1920 1,328,657 Fish, Jr Jan. 20, 1920 1,456,809 Lyon May 29, 1923 1,687,900 Shinn Oct. 16, 1928 1,778,079 Kristensson Oct. 14. 1930 2,060,902 Stamm Nov. 17, 1936 2,273,039 Hudson Feb. 17, 1942 OTHER REFERENCES "Drying Lumber by means of superheated Steam," etc., by H. D. Tiemann. Report of Forest Products Laboratory. Revised August 15, 1931 (page 1 iaon1y), 

